Wide bandgap (“WBG”) semiconductors are emerging as materials capable of driving continued device performance enhancements for many years to come. These materials are typically defined as exhibiting an electronic bandgap greater than 2 eV. Silicon Carbide (SiC) has been studied for decades, but recent developments have firmly established SiC commercial products in optical, RF, and power components. With extremely high thermal and chemical stability, and electrical performance, WBG devices are used for high frequency, high temperature, and high power applications. Examples of WBG semiconductors are gallium nitride (GaN, EG=3.4 eV), aluminum nitride (AlN, EG=6.2 eV), and silicon carbide (SiC, EG between 2.2 to 3.25 eV depending on polytype).
Wide bandgap devices afford significant performance advantages compared to silicon semiconductor devices. Silicon carbide Schottky diodes are finding widespread use because of their fast switching speed (low trr), low stored charge (Qrr) and low forward conduction loss. However, wide bandgap devices (e.g., silicon carbide) have poor and erratic avalanche capability due to starting substrate quality. Avalanche breakdown is a form of electric current multiplication that can allow very large currents to flow within materials which are otherwise good insulators. Avalanche breakdown can occur within solids, liquids, or gases when the voltage applied across the insulating material is great enough to accelerate free electrons to the point that, when they strike atoms in the material, they can knock other electrons free. This phenomenon can be quite useful in semiconductor diodes such as the avalanche diode and avalanche photodiode, but in other situations, such as in MOSFET transistors, avalanche breakdown can destroy the device. When avalanche breakdown occurs within a solid insulating material it is almost always destructive. However, improvements in wide bandgap avalanche capability is desired.
A common avalanche diode application is protecting electronic circuits against damaging high voltages. The avalanche diode is connected to the circuit so that it is reverse-biased. In other words, its cathode is positive with respect to its anode. In this configuration, the diode is non-conducting and does not interfere with the circuit. If the voltage increases beyond the design limit, the diode undergoes avalanche breakdown, limiting the harmful voltage. When used in this fashion they are often referred to as clamp diodes because they “clamp” the voltage to a predetermined maximum level. Avalanche diodes are normally specified for this role by their clamping voltage VBR and the maximum size of transient they can absorb, specified by either energy (in joules) or I2t. Avalanche breakdown is not destructive, as long as the diode is not allowed to overheat.
Paralleling a simple zener diode with the wide bandgap device can result in the injection of minority carriers in the wide bandgap device, thus degrading the switching performance of the wide bandgap device. As discussed in U.S. Pat. Nos. 6,144,093 issued to Davis et. al. (“Davis”), and U.S. Pat. No. 5,544,038, issued to Fisher et. al. (“Fisher”), when a MOSFET or other MOS gated semiconductor device is used in switching applications in which the gate of the device is repeatedly turned on and off, transient currents can flow through the body diode of the device when the device is turned off, increasing the turnoff time of the device. One solution has been to place a Schottky diode in parallel with and oriented in the direction of the body diode to provide a faster path for the flow of transient currents. Typically, the Schottky diode has a forward voltage drop of about 0.4 V, whereas the body diode typically has a forward voltage drop of 0.7 V. The Schottky diode thus prevents the body diode from conducting because the Schottky diode generally has a lower forward voltage drop than the body diode. However, unlike one embodiment of the present invention, Davis requires at least two wirebond connections and a third connection, and Fisher requires a transistor.
Clamp protection for wideband gap devices that prevents injection of minority carriers in the clamping device which results in degrading the switching speed advantage is desired.
Further, clamp protection that is easy to implement monolithically and provides uniform field distribution for good wide bandgap device termination edge breakdown voltage (“BV”) is desired.
Further, clamp protection that has offsetting temperature coefficients of forward and reverse junctions to provide minimal BV temperature coefficients is desired.
Even further, clamp protection that allows avalanche capability to be independently scaled is desired.